If the leak is not from the hoses and not from the engine, then the tank may be leaking.
Tanks leak for several reasons
Water gets under the tank and the tank corrodes.
A dissimilar metal comes in contact with the tank, the tank corrodes. This is likely an errant screw.
Water inside the tank allows anerobic bacteria to grow. These sulfur digesting bacteria form colonies and their waste products are acidic. The acid is trapped within the colony and corrodes the aluminum. Interesting phenomena, look up "Microbiologically Induced Corrosion."
If the tank is leaking, its time for a new tank.